1. Field of the Invention
The present invention relates to a color cathode ray tube used for color displays for TV, and in industry, as well as for other special uses.
2. Description of the Prior Art
The face glasses of these color cathode ray tubes are classified by ranks of light transmission, into a clear face having visible light transmission of at least 75%, a gray face having visible light transmission of 60 to 75% and a tainted face having a visible light transmission of upto 60%. In the color cathode ray tubes, gray faces or clear faces having high light transmission have been mainly used in view of brightness. Tinted glass having less light transmission is suitable for absorbing outer light to improve contrast. In general, the output of the fluorescent screen of a color cathode ray tube is not high enough to give the desired light when the light transmission is low. Therefore, the clear or gray glass has been mainly used. In order to achieve high brightness and high contrast, various improvements have been studied. One of the improvements is a black matrix and recently a pigment phosphor has been used.
For example, a shadow mask type color receiver comprises a face glass part (1) on an inner surface of which a phosphor is coated; a funnel part (2) connected to the face glass with a glass frit; a neck part in which an electron gun is held, and a shadow mask (4) placed near the fluorescent screen inside of the vacuum envelope as shown in FIG. 1.
The shadow mask (4) has the function of a color selecting electrode so as to project electron beams (5B), (5G), (5R) which pass at specific angles through the holes of the shadow mask (4) which to correspond to phosphor dots (6B), (6G), (6R) for luminous colors formed on the screen which the electron beams reached.
The gray glass having the light transmission spectrum shown by the curve (II) in FIG. 4 has been mostly used as a face glass part (1) of a color cathode ray tube having such structure. The black matrix type face glass prepared by filling spaces between the phosphor dots (6B), (6G), (6R) of blue green and red, with a black paint as a light absorption material (7) as shown in FIG. 3 has been mainly used. The clear glass having high light transmission as the curve (I) in FIG. 4 has been mainly used as the face glass. The light of the phosphors is emitted as much as possible, out of the tube by using the clear glass. On the other hand, the black matrix layer is formed to absorb outer light and light which is reflected on the face glass surface to return the same to the fluorescent screen thereby improving contrast.
Recently, it has been proposed to use pigment phosphors prepared by incorporating a pigment in a phosphor so as to selectively increase the percent reflectivity of the emitted color of each phosphor thereby further improving contrast. The characteristics have been improved by these proposals however, the improvement is not enough level to satisfy the commercial requirements for higher brightness and higher contrast. When the tube is used for the terminal display of a computer, figures, symbols or graphs are displaced on a part of the screen in many cases so that the ratio of the luminous area to a total effective screen area is only upto 10% in many operations. An important problem is to improve the contrast without deteriorating brightness.